Python RegEx | Regular Expression Tutorial with Examples
The RegEx of the Regular Expression is actually a sequene of charaters that is used for searching or pattern matching.
Python has module re for matching search patterns. It comes inbuilt with Python installation. You just need to import and use it.
Let’s see this simple Python regex example.
Search if there is at least two consecutive English uppercase letters.
import re
print(bool(re.search("[A-Z]{2}", "cdABp")))
#True
print(bool(re.search("[A-Z]{2}", "1Z")))
#False
print(bool(re.search("[A-Z]{2}", "A1P")))
#False
Table of Contents
RegEx Functions
Python re modules offer the following functions for pattern matching. You can use them for different purposes.
| Function | Description |
|---|---|
| search | returns match object if match found |
| findall | returns list of all the matches from the given string |
| split | returns list of string where it has been split for each match |
| sub | returns string by replacing all the matches |
RegEx Sets
Just like the English uppercase letters example, we have seen above, there are other different sets you can use in Python regex.
| Set | Matching Characters |
|---|---|
| [A-Z] | set which returns a match for any character between ‘A’ to ‘Z’ (uppercase letters) |
| [a-z] | set which returns a match for any character between ‘a’ to ‘z’ (lowercase letters) |
| [0-9] | set which returns a match for any character between ‘0’ to ‘9’ (numeric number) |
| [A-Za-z] | set which returns a match for any alphabetic character (includes both uppercase and lowercase letters) |
| [A-Za-z0-9] | set which returns a match for any alphabetic or numeric character (includes both uppercase, lowercase letters and numbers) |
| [apx] | set which returns match if one of the character (‘a’, ‘p’, ‘x’) are present |
Special RegEx Characters
| Character | Description | Example |
|---|---|---|
| [] | to define set of characters | [A-Z] match all the uppercase letters |
| {} | to match the exact number of characters | {2} match only two characters |
| ^ | to check the prefix (starts with) | “^CSE” string start with “CSE” |
| $ | to check the postfix (ends with) | “stack$” string ends with “stack” |
| * | to match zero or more occurrences | pos* match the string contains “po” followed by zero or more “s” characters |
| + | to match one or more occurrences | pos+ match the string contains “po” followed by one or more “s” characters |
| | | or operation | “CSE|stack” match string contains either “CSE” or “stack”: |
| () | form a group | |
| . | to match any character (except newline character) | “CS..ack” match the string containing “CS” followed by any other two characters and then “ack” |
| \ | to match special sequence | “\d” match all digit characters |
Python RegEx Coding Examples
1. search() function
Search the first occurrence of “ing” in the given string.
import re
match_obj = re.search("ing", "CSEstack Programming Portal")
print(match_obj.start())
print(match_obj.end())
Output
17 20
Explanation
- The search function returns a match object.
- If no match found, it returns the NoneType object.
- You can use the
start()andend()methods of match object to get the start and end index of the matching pattern. - You can also convert a match object into the
True/FalseBoolean values usingbool()method. ReturnTrueif match found, elseFalse.
2. findall() function
Find all the matching which contains character “r” and followed by two any other characters.
import re
match_list = re.findall("r..", "Programming Portal- Keep Coding")
print(match_list)
Output
['rog', 'ram', 'rta']
Explanation
- Method
findall()returns Python list object. So, you can perform any operations on matches that we do with the Python list.
3. split() function
Split the given string by occurrence of character ‘a’ or ‘g’.
import re
match_list = re.split("a|g", "CSEstack Programming Portal")
print(match_list)
Output
['CSEst', 'ck Pro', 'r', 'mmin', ' Port', 'l']
Explanation
- Like
findall(),split()method also returns Python list.
4. sub() function
Replace all the occurrences of substring “cse” with “CSE”
import re
match_string = re.sub("cse", "CSE", "Learn coding from csestack.")
print(match_string)
Output
Learn programming from CSEstack.
Explanation
- Method
sub(), returns the Python string.
These are the simple examples to demonstarte different reguylar explression and methods.
Regular expressions are extremely useful in different fields like data analytics or projects for pattern matching.
Python RegEx Questions asked in Interview
Here are some of the examples asked in coding interviews. Take them as coding exercise and practice solving them.
- Write a regular expression to check the valid IP addresses.
(This was asked in Juniper coding interview.) - Based on the given criteria, find all the Visa, MasterCard and American credit cards.
(This was asked in Byju’s coding interview.) - Find all the PAN card numbers from the given paragraph.
(This was asked in the BrightMoney coding interview.)
This is all about Regular Expression in Python (Python RegEx). Hope you find this tutorial useful for your learning.
